Plasma arc welding or plasma welding is, as taught by the document "Plasma Arc Welding", Chap. 10, pages 330-350, 8th edition 1991, incorporated here by way of reference, a metal fusion welding process which involves heating the metal by means of an electric arc which forms between a non-consumable electrode and the metal workpiece or workpieces to be welded (transferred arc) or, depending on the case, between the electrode and the nozzle of the plasma torch (non-transferred arc).
An ionized gas flow delivered by the plasma torch serves, on the one hand, as a gas shield and, on the other hand, to transfer the heat generated by the electric arc to the workpiece to be welded and, optionally, to channel the electric arc between the non-consumable electrode and the workpiece to be welded.
An inert gas, such as argon or a mixture comprising such a gas, for example an argon/helium mixture, may be employed as the plasma gas. Usually, plasma arc welding processes are used to weld "carbon" steels or stainless steels in a single pass, that is to say in a single run, and without any special preparation beforehand of the steel workpieces to be welded, this being the case for thicknesses ranging up to 10 mm.
Thus, in "keyhole" welding mode, especially by dint of a high energy density and of constriction of the arc, a hole is formed in line with the weld joint and the flow of plasma gas penetrates reliably through the entire thickness of the material.
Moreover, aluminum and aluminum alloys are conventionally welded by inert-gas-shielded arc welding processes, such as the TIG (Tungsten Inert Gas) or MIG (Metal Inert Gas) type processes rather than by using plasma arc type processes.
However, there has for some time now been a strong demand from industry, particularly from the aeronautical and aerospace industries, for plasma arc welding processes allowing aluminum and aluminum alloys to be effectively welded, in particular using automatic welding.
Thus, plasma arc welding in "keyhole" mode, which was firstly used widely for welding carbon and stainless steels, has been extended to the welding of aluminum and of aluminum alloys.
The document "Plasma Arc Welding of Aluminum Gas Containers", by H. Fostervoll et al., pages 367-375, incorporated here by way of reference, describes a process for the plasma arc welding in "keyhole" mode of two aluminum hemispheres or half-containers so as, after joining and welding them together, to form a gas container 352 mm in diameter, the aluminum walls of which have a thickness of 8 mm. This process uses a variable-polarity welding set of the HOBART.TM. VP-300-S type, a welding control system of the ISOTEK.TM. type and 99.99% pure argon as the plasma gas.
Furthermore, the document "Variable Polarity Plasma Arc Welding on the Space Shuttle External Tank", by A. C. Numes et al., Welding Journal, September 1984, pages 27-35, describes a variable-polarity plasma arc welding process used for NASA in order to produce the external tanks of the American space shuttle. This document particularly stresses the low cost of the plasma arc process, compared with conventional TIG or MIG processes, when welding aluminum, given that this process especially saves having to pretreat the workpieces to be welded.
This is because, unlike steel, aluminum and its alloys require, before welding, a prior preparation or pretreatment in order to remove therefrom the oxides and other contaminants (dust, grease, etc.) that are likely to cover them. Usually, this pretreatment is carried out by chemical pickling or mechanical descaling, such as brushing, of the said aluminum workpieces, which correspondingly increases the production costs.
However, it has been observed that by varying, over time, the polarity of the current used during the arc plasma welding it was possible to effect a kind of descaling of the surface of the material by the plasma flow, prior to the actual welding of the material.
More specifically, variable polarity is a form of alternating current having an asymmetric, rectangular waveform which can be controlled in terms of period and in terms of amplitude.
Usually, the current is varied cyclically from a background current (Id) or descaling-phase current, maintained for a descaling time (Td), to a peak current (Iw) or welding-phase current, maintained for a welding time (Tw).
In general, a descaling time (Td) of duration less than the welding time (Tw) is chosen, but with a greater amplitude of the current during the descaling phase than that during the welding phase.
This is because it has been found that the lifetime of the tungsten electrode depends on the Tw/Td ratio.
Furthermore, the set-back of the electrode in the nozzle of the welding torch, the torch height with respect to the workpiece or material to be welded, and the plasma gas flow rate play a not insignificant role in the dynamics of the flow.
Thus, in keyhole-mode welding, it is necessary to control, that is to say limit, the flow rate of the plasma gas in order to prevent any adventitious cutting of the workpiece. This may especially be accomplished by choosing an electrode diameter as small as possible so as not to cause too rapid a gas flow.
On the other hand, a large set-back distance of the electrode in the nozzle and/or a higher current signal frequency make it possible to increase the stiffness of the plasma column and therefore to decrease the supply of electrical energy.
Variable-polarity welding also has the advantage of resulting in low porosity in the weld bead.